To be fair, the $3999 Mac they are talking about is the highest configuration of the top iMac model - 24 inch screen. I'm guessing the hardware on the Dell isn't anywhere near as good. For it to be a fair (hardware) money comparison, the hardware needs to be as close as possible. About a year ago when I compared laptop models to assist my grandparents with their decision to by a laptop computer to replace their WebTV, the then MacBook model (the white plastic one that is the lowest model now) actually beat Dell and HP's configurations for hardware.
And while this may be true today (I didn't check it), if custom hardware is important to you, I would by no means recommend a Mac. Their compact, sleek design makes it nearly impossible to upgrade the hardware. But on the plus side, because Apple only has to design the OS for one set of hardware, they are more backwards compatible meaning you don't need to upgrade the hardware to get the newest Mac OS X to run.
As far as the actually Operating Systems go, I believe it's a toss up. User preference is going to play a lot into it, as is desired use. Obviously gaming, though getting better, still isn't quite up to snuff on the Mac (primarily because game makers to target to the Mac audience). But in my opinion for general purpose, "office" or "education" uses, the Mac has the PC beat. There are a number of things that make the Mac more conducive to productivity... but of course this is heavily based upon my opinion. I also haven't used Windows 7, but the screenshot's I've seen make it look pretty cool so perhaps it will give Apple some better competition.
Currently, I'm running Windows XP on my main computer (and Ubuntu Server on my server
), but lastnight I just made the big step and finally ordered my new MacBook. I'm pretty excited, and I've needed a laptop for a long time so I'm looking forward to Wednesday-Friday when it should come. I'll be sure to post back here with new little things I've found after using the Mac as a main environment. Before now I've only used Macs my friends/family has owned so I'm sure there is more to pick up on.